Microwave carrier telephone system



Aug. 18, 1953 S. D. ROBERTSON MICROWAVE CARRIER TELEPHONE SYSTEM Filed Feb. 21, 1948 Sheets-Sheet 1 Aug. 18, 1953 s. D. ROBERTSON 2,349,539

MICROWAVE CARRIER TELEPHONE SYSTEM Filed Feb. 21, 1948 s s neepssheet s FILTER M/CMMVE /A/r/ENTOR 5V S. D. ROBERTSON www? A TTOR/VEV Patented Aug. 18, 1953 MICROWAVE CARRIER TELEPHONE SYSTEM Application February 21, 1948, Serial No. 10,023

17 Claims.

This invention relates in general to the transmission and reception of radio waves, and more particularly to radio carrier signaling systems.

In accordance with certain prior art systems, such as disclosed in Patent 1,771,148 to C. A. Sprague, July 22, 1930, the wire circuits used in conventional telephone systems to conduct electrical message currents between central and satellite stations are replaced by directed beams of microwave radio energy which are initiated in the central station, and intercepted and reflected at each of the satellite stations. Messages are transmitted from the central station by modulation of the transmitted carrier beam; and from the respective satellite stations by modulation of the reflected carrier beam.

It is therefore the broad object of this invention to provide for improved signal transmission and reception in directional radio telephone systems.

A more specific object is to provide improved methods and apparatus for establishing contact between stations in systems of the aforesaid type.

Another object is to provide for distance discrimination between satellite stations located in the same direction from the central station.

Still another object of the invention is to reduce signal distortion due to phase fluctuation in a system of the aforesaid type.

In carrying out the objects set fort-h above, in accordance with the present invention, a microwave beam telephone system of the general type disclosed includes a rotating central station carrier beam which repeatedly scans a plurality ci outlying satellite stations to locate calls, and

which Xes its orientation in the direction or a particular calling station for the mutual transmission of speech and ringing signals. When a subscriber at a particular satellite station wishes to make a call, he impresses a modulating calling signal on the reilected carrier beam which actuates control means in the central station to fix the orientation of the carrier beam in his direction whereby it serves as a medium for the transmission of speech signals between the conn tral stat-ion and the calling satellite station.

In order to permit intercommunication between satellite stations, duplicate sets of beam transmitting and receiving apparatus and suitable switching means are provided at the central station.

In accordance with a modification of the system described in the foregoing paragraphs, the transmitted carrier beam is impressed with a repetitive saw-tooth frequency mOdulatiOll, and

the signal-modulated echo beam combined with a component of transmitted carrier to produce an intermediate beat-frequency wave. Such a system provides two particular advantages; namely, it inherently reduces fluctuations in the level of the received signal due to slight shifts in the carrier frequency; and it permits discrimination between subscribers in the same direction but at different distances from the central station by means of frequency multiplex reception.

In accordance with another modiiication of the invention, the transmitted beam and the reected beam are differently polarized, thereby reducing the amount of sidetone present in either the central or satellite stations.

Other objects and features of the invention will be apparent from a study of the detailed speciication hereinafter and the attached drawings in which:

Fig. 1 shows a schematic drawing of a central station apparatus in accordance with the present invention, including an equalizer circuit for eliminating phase distortion, such as described in detail in Fig. 2 of application Serial No. 10,148 of W. J. Albersheim, filed at even date herewith issued as United States Patent 2,556,669, June 12, 1951;

Fig. 2A shows a modiiication of the central station apparatus of Fig, 1 to include means for impressing a saw-tooth frequency modulation on the transmitted carrier beam, and a frequencydivision multiplex receiving circuit adapted to distinguish between satellite stations at different distances from the central station;

Fig. 2B is a graph illustrating certain theoretical principles of the embodiment oi Fig. 2A;

Fig. 3 shows a schematic drawing of a typical satellite station in the system of the present invention; and

Fig. 4 shows a schematic drawing of a system in accordance with the present invention in which the transmitted and reiiected beams are differently polarized.

A typical embodiment of the microwave telen phone system of the present invention which will now be described comprises a central station equipped as shown, for example, in Fig. 1 and a plurality of subscribers stations equipped as shown, for example, in Fig. 3. Both the central and subscribing stations have ringing circuits and speech transmission and receiving circuits such as are customary in conventional wire transmission telephone systems. The present system is so designed that during the interval between calls, a directed microwave carrier beam enel-ated at the central station is periodically "otated through a scaning arc in a horizontal .ne thereby contacting in succession the sub- `c lbers stations that lie within its range. lf subscriber desires to make a call, he lifts his receiver 01T the hook-switch in the usual manner. When the scanning beam contacts his antenna this operation impresses a iixed frequency ringing signal on the portion of the beam renected back to the central station where the received signal actuates means to cut off the scanning drive and fix the orientation of the carrier beam in the direction of the calling station. Completion of this operation is indicated by the lighting of a signal lamp in the central station, whereupon the central station operator transmits a ringing signal over the beam which disconnects the calling subscribers ringing equipment, and conditions the system for the transmission and reception of speech signals between the central and calling stations. Assuming that the calling subscriber wishes to contact another subscribers station, he gives the appropriate number to the operator, whereby a second carrier beam at the same central station is directed toward the called station, and impressed with a ringing signal to ring the called subscribers bell. Upon receiving a return ringing signal from the called station, the operator plugs jumpers between the respective speech transmitters and receivers of the two central station circuits which operation conditions the system for speech transmission between the calling and called subscribers station. When either of the subscribers hangs up, that fact is indicated to the central operator by a ringing signal which lights a signal lamp, whereupon the operator makes the proper disconnections and reestablishes scanning operation of the beams to locate calls. A single central station preferably includes several identical transmitting and receiving circuits, so that calling stations are continuously being located, and a number of pairs of subscribers stations may be engaged in conversation at one time. To obviate the necessity of providing a plurality f speech modulating equipments at the central station, suitable talking and listening switches of a type well known in the art such as for example voice operated switches may be provided to transfer the speech modulations received from a subscriber on one central station beam for transmission to a subscriber on another central station beam.

Referring to Fig. 1 of the drawings, which shows one form of a transmitting and receiving circuit in the central station, a conventional microwave oscillator Il of one of the types known in the art, which is adapted to produce oscillations at a power of the order of 50 watts having a wavelength of, for example, 3.2 centimeters is connected through a conventional wave guide to a microwave amplier-modulator |92, which may be any of the types well known in the art. The amplifier-modulator |02 is connected to the arm IBS of a hybrid junction IM commonly known as the Magic T, which comprises a four-arm E-H junction such as shown for example in Fig. '7 of application Serial No. 581,285 led on March 6, 1945 by W. A.. Tyrrell, issued as United States Patent 2,445,896, July 27, 1948.

In the hybrid junction |04, the two conjugate branches I and l which are coplanar with the arm |03 are constructed to present substantially equal impedances at the junction, whereby Si) I no energy entering the junction through the arm 03 can reach the output arm |42 in the electric plane.

The hybrid junction |94 therefore functions to separate the carrier wave output of the amplifier-modulator |32 into two substantially equal components, one of which is conducted into the receiver through the wave guide arm |ii5 and the connected coaxial line |50 to act as a beating wave, and the other of which is conducted into 4the antenna feed circuit. IThis comprises the wave guide rotary joint or choke Ici', such as disclosed, for example, in application Serial No. 464,333 to A. G. Fox, filed November 3, 1942, issued as United States Patent 2,438,119, March 23, 1948, the fixed portion of which is connected to the guide arm we, and the rotatable vportion of which is connected to the hollow cylindrical feed wave guide |438. The rotatable feed guide |638 is connected to the antenna feed horn ||i which is disposed at the focus of the parabolic reflector i12 which is dimensioned and constructed in accordance with well-known practice to produce a substantially parallel directed of carrier radiation, the allowable spread and the power of the transmitted beam being a function of the distances between central and subscribing stations which are here assumed to be of the order of several miles.

The antenna ||2 and associated feed |I| is adapted to be rotated by means of a meshed gear rive Il@ mounted on the wave guide pipe IEEE, which is driven to rotate by the drive shaft il connected to motor I I4. A handle I Mia attached to the motor Ilfl provides for manual rotation of the antenna assembly. The motor I4 is connected to the alternating current power source I|3 through a conducting circuit which includes the armature and contacts and lila of the relay H5 and the manually operated switch ffl. Also connected to the alternating current source ||3 through the armature and contacts HS and H811 of the relay lib is the solenoid i which functions to actuate the armature |25 connected to the pivoted member |25, to control the engagement of the brake shoe IZtc with the brake drum lts mounted on the hollow gui-:le itt. In addition, the alternating current source H3 energizes the signal lamp |23 through a circuit which includes the armature and contacts I I9 and Htc of the relay Ile.

A circuit providing signal currents to :nodulate the outgoing carrier waves originating in the microwave oscillator ISI is connected to the microwave amplifier-modulator |92. This circuit includes a conventional telephone transmitter |38 energized by the direct current power source |39, and connected in series with the primary coil of the transformer IM, so that currents representing speech fluctuations are conducted to the input of the low frequency ampli'- iier |36 through the secondary of the transformer lli@ and a jack and plug connection ISA-|35. The input of the amplifier |35 is also connected to receive a superaudible ringing current from the oscillator ESI, which may assume any of the forms well known in the art adapted to produce oscillations of the order of, for example, 20 or 30 kilocycles per second. The connecting circuit between the ringing oscillator |3i and the amplifier |35 includes the normally open ringing push button |32, the transformer E33 and the jack and plug connection i3d-|35.

A circuit for receiving and demodulating the carrier waves received through the antenna assembly 2 includes the cylindrical rotatable Wave guide |533, the wave guide rotary joint |01, to which is mechanically coupled the fixed Wave guide arm |86 of the hybrid junction |84, which is so constructed that the receiving waves separate into two components, one of which is absorbed in the transmitting circuit, and the other of which passes into the receive wave guide arm |42.

The wave guide arm |42 is connected to feed into the magnetic plane of a second hybrid junction |43, which is substantially similar to the junction |64, and which is terminated in the electric plane in an impedance matching stub line |4|, whereby the incoming modulated Waves are separated into two equal components passing into the wave guide arms |44 and |45, wherein they are mixed with unmodulated components of carrier wave which are respectively 90 degrees out of phase. -These latterV components are 'derived directly from the output of amplifier-modulator |132 through the arm |65 of the hybrid junction |64 and the coaxial attenuation line |50, which feeds into the respective wave guide arms |44 and |45 through separate connecting wave guide arms 548 and |453 which differ in length by a quarter of a wave guide wavelength. The attenuation line |50, the guide arms |44 and |45 terminate in the crystal detectors |46 and |41, the outputs of which are separately fed into the quadrature circuit |53 which serves to combine these components in such a manner as to reproduce a signal current which is substantially free from phase distortion .which is caused by slight frequency or phase shifts on the output of the microwave oscillator |0|. The quadrature circuit |53 is described and shown in detail with reference toy Fig. 2 of application Serial No. 10,148 to W. J. Albersheim, filed at even date herewith.

For operation at a wavelength of 3.2 centimeters for example, all of the connecting wave guides preferably have rectangular cross-sectional dimensions of, for example, 1.016 centimeters by 2,285 centimeters. Except where specied otherwise, the lengths of the connecting wave guides are determined by convenience.

The circuit which responds to the demodulated speech and ringing currents emerging from the output of the quadrature circuit |53 includes the conventional audio-frequency amplier |54, the output of Which is connected in parallel through the plug and jack combination |55|56 to the telephone receiver |51 and through the transformer |58 to the calling signal response ciroint connected to actuate the relay ||6. The calling response circuit includes the filter |59 tuned to the particular ringing frequency, and the conventional rectier |60, the output of which is connected to energize the relay |6 thereby causing operation of its armatures ||1, I8, and i9 to respectively break contact |1a cutting 01T the antenna motor, make contact ||8a applying the brake |2|5a to the drum |09 on the rotating guide IBS, and make contact |9a lighting the signal lamp |23.

In the general case, the location and identification of a particular calling station is indicated to the central station operator by the antenna direction indicator |63, which is equipped with a pointer adapted to move along a scale in synchronism with the rotational motion of the antenna. This is accomplished by a mechanical connection between the gear drive l0 and a conventional self-synchronizing motor |62, the output of which is connected to move the pointer 6 of the indicator |53. Prior to the operation of the system, the scale on the direction indicator |63 is calibrated in terms of the angular position of each of the subscribers stations on the horizontal scanning arc through which the antenna moves.

Moreover, as pointed out hereinbefore, the problem of intercommunication between subscribers stations is solved in the general case by providing a plurality of identical beam transmitters and receivers at the central station, which are either equipped with separate speech modulating circuits or with suitable talking and listening switches operating in conjunction with a single speech modulating circuit.

However, a special problem arises with regard to identification of and intercommunication between subscribers stations located at diierent distances but in the same direction from the central station. 1

As pointed out with reference to Fig. 1 hereinbefore, in a homodyne system, level uctuation occurs in the received signal as a result of slight shifts in frequency in the carrier-wave oscillator. In the system of Fig. l, this distortion is compensated for in the quadrature equalizer circuit |53 disclosed in the application of W. J. Albersheim supra.

The presence of such level fluctuations may be explained theoretically as follows.

Let a carrier wave,

et=ec COS wt (1) be radiated by the central station. It arrives at the satellite station located at a distance D with an attenuation A and a delay time TED/c (2) c being the speed of light in the intervening atmosphere. After passing through subscribers audio frequency modulator it has approximately the form es=Ae0(1-|m cos at) cos (wt-WT) (3) m being the modulation factor and a the instantaneous frequency. The subscriber-modulated carrier Wave es is reflected back to the central station where it undergoes a second `attenuation A and time delay T and is received as an echo.

If this echo is mixed with a component of the transmitted carrier en and passed through a square-law detector, the derived low frequency component is where lc is a constant of proportionality. This has maximum absolute values when 2wT=N1r (7) It is zero when Assuming a constant delay time T, gain uctuaemesse tionf fremfmaximum. to .extinction .occursf fmy a Bequemer-shift.in-which 1r AwrZ In this case,

Aw ylf-118D (lo) Where l is the wavelength.

Bylway ofillustration, :iet the distance between .the central station -anda .selected subscriber'sta- Ytion be; ten -miles .which equals lsmall Vfraction of 1.0-7, which is impractical.

In compensating for uctuations in theSgHalevel,` such -as the aforesaid,l the embodimentof Fig. 2A, to be described presently, employs a difw ferent expedient than vthe lquadrature equalizer circuit of Albersheim supra. In accordance with the modification ofliiig. 2, .a repetitive saw-tooth frequency modulation is impressed in the transmitted central stationfcarriergbeam; and the arn- -plitudefmodulatedpecho waverefiected back. to the central .station from thesubscriber or satellite station is beaten with a component of transmitted carrier to producean intermediate frequency wave. Such a system has the added iean ture ofproviding for discrimination between subscribers-located at different distances but in the samedirection from the central station by an arrangement for frequency-division multiplex reception.

The operation-ofl the system will be better understood from a study of the following theoretical. considerations.

Referring'to Equation 6 above, it is'seen that the output cfa homodyne square-law modulator -is'ftheeproduct of the desired signal multiplied by 'a factor cos ZNT.

In the embodimentAo-f Fig. 2A, as VApointed out, the carrier frequency e is modulated by a sawtooth wave of frequency u. During nearly the entire period tionis removedby a-:demodulator Yor :detector andthe echo carrier, wgfi-/iwut-ZT).

which may beA any non-linear typesuchl as, for

example, lasquarev-law detector and yaV low-pass iilter connectedv thereto, the' resultant product. is proportional to lei-mgcos at. If desired, the direct current componentfmayy be removed by an .additional raudioV frequency amplifier.

The'foregoing relationships are illustratedin Fig. 2B wherein-the solid saw-tooth line inthe upper portion indicates the transmitted carrier frequencies, and thef'dotted line indicates the echo carrier -frequency received by the central station `which is delayedby` 2T. This delay time is, in most practical cases, smaller than the sawtoothg repetition period .although thisv is` not necessary.

During the time interval thetransmitted carrier-frequency is wo-l-Aw/it -Hence, the. carrier difference I` is equal to Molti-as derived in Equation 16. During the interval 2T in which the transmitted frequency'has swept back, one :saw-tooth frequency swing, that is An the echo frequency is still increasing; the transmitted carrier. frequency is therefore wov-l-Aw (ut-f 1) Hence the :Thisirequency y difference.which may `also be called beat frequency orheterodyne frequency, is .shown .as adash-dotted stepped line in the lower part ofFig. 2B. The edges of this stepped line show a slightdeviationfrom vertical due to the, fact that. the saw-tooth back sweep, although extremely fast, requires Va nite time interval T.

Theapparatusof Fig. ZA-is designed to produce a .saw-tooth frequency modulated carrier beam which, when reflected and amplitude modulatedatlthe satellite station, is beaten with a component of frequency modulated transmitted carrier wave to produce an intermediate frequency wave l in accordance with the foregoing theory.

W ith the exception of the saw-tooth` generator 2219, that part ofthe embodiment disclosed in Fig.

2A.which is not enclosed in the dotted line box ABCD, compriseselernents substantially similar inv structureand function ,to corresponding eletooth envelope preferably of superaudible lfrequency which increases linearly from a minimum. to a maximum, returning again to minimum with `a rapid fly-back.

The frequency modulated output from the voscillator 28! passes through the amplier-modu lator vv2.62 .which is connectedrso that its output passes into the hybrid junction Zilli, which is substantially similar-to the hybrid junction me of Fig. 1, and is constructed with conjugate arms v295 -and 26S adapted to present substantially -equal impedancesat;thefjunction. The armr205 istv connectedl to supply #a beating-,carrier compo- 9 nent to the receiving circuit through the coaxial attenuator 265 and connecting arm 266. The attenuator 265 can take the form of a wave guide attenuator or any other attenuator known to the art. The arm 206 is connected to impress nergy on and receive energy from the antenna The wave guide arm 261 is so connected to the hybrid junction 204, that it receives a component of the modulated incoming carrier wave, which is beaten with a substantially greater component of unmodulated carrier wave supplied through the connecting arm 266, and the coaxial attenuator 255. The wave guide 251 is terminated in a crystal detector 268 which serves as a demodulator of the beat frequency wave and which is connected to a frequency division multiplex circuit, which functions to separately receive the speech and ringing signals from the satellite stations in the same direction but at different distances from the central station.

The frequency division multiplex circuit, which is enclosed by the dotted line box A, B, C, D, in Fig. 2A, will now be described n detail.

rIhe output circuit of the crystal demodulator 268 is connected to a plurality of conventional intermediate frequency amplifiers 210a, 2101 210e, etc. connected in parallel, and each tuned to amplify a different range of beat frequency waves from the demodulator 268. Alternatively, a single broad-band intermediate frequency amplier may be used, and the output divided into a desired plurality of channels by conventional band-pass filters.

Each of the intermediate frequency amplifiers 21011, 2101i, 216C, etc., or alternatively, different frequency channels from a single intermediate frequency amplifier, is respectively connected to one of audio detectors 21161, 2111), 211e, the output terminals of which are connected to the respective audio amplifiers 254g, 2541) and 254C.

The individual speech receiving circuits for the different frequency bands, respectively include audio amplifiers 2540 2541), 25de, jacks 255er, 2551), 255C which operate upon insertion of the plug 256 to energize the conventional telephone receiver 251.

For reception of the ringing signals, the separate audio outputs of the amplifiers 254m, 2541), 254e are fed into the respective transformers 258s, 25819, 258C. These are connected through to the low-pass filters 259e, 25913, and 25 9c, which may be tuned to different frequency identifying signals to the respective rectiers 2651i, 26th, and 26Go to energize the relays 212e, 21212, and 212e.

The relays 21211, 21273 and 212e respectively operate to actuate armatures 213a, 21319, 213e which close corresponding contacts 214e, 2141), 214e to energize the relay 2 I 5, corresponding to the relay l I5 of Fig. 1 which functions to operate the motor control circuit and brake to halt the antenna scanning mechanism. Upon operation, the relays 212a, 21213, 2120 also actuate the respective armatures 215m, 2151), 215e to engage respective contacts 216m, 2161i, 215e, thereby closing the circuit of the battery 218 to energize the proper one of the signal lamps 211er, 2111), 211C, whereby the particular calling satellite station is identified to the central station operator.

Discrimination between outgoing signals from the central station destined for different satellite stations is achieved in a manner disclosed with Vreference to Fig. 5A of application Serial No.

10,148 to Albersheim, filed at even date herewith, whereby a bandpass filter tuned to a separate microwave frequency can be inserted in the receiving circuits of each of the satellite stations, as shown in Fig. 3, and described hereinafter, making a particular subscribers receiving equipment responsive to only a part of the frequency band over which the saw-tooth FM of the transmitter sweeps. Let, for example, subscriber A be limited to the lower half of the maximum sweep range of the transmitted carrier wave and subscriber B to the higher half of that range. Then discrimination in favor of A or B, respectively is obtained at the central station by limiting the transmitter FM sweep to the lower or higher half of the maximum swing, as desired. rL his is preferably achieved by biasing the control voltage applied to modulate the output of the microwave oscillator, in a manner disclosed with reference to Fig. 5b in the Albersheim application supra.

Fig. 3 of the drawings shows a typical subscribers circuit which is adapted to perform the dual functions of receiving the speech and ringing signals impressed as modulations on a beam of carrier waves from the central station, and of transmitting the subscribers speech and ringing signals to the central station by impressing them as modulations on the reiected portion of the unmodulated carrier beam supplied from the central station.

The subscribers circuit of Fig. 3 comprises in combination a paraboloidal antenna reflector Si'iii at Whose focus is disposed a wave guide feed horn 311. The aforesaid units are shaped and climensioned in accordance with well-known practice to provide for the reception and retransmission of the substantially parallel directed carrier beam from the central station, which by way of example has been assumed to have a wavelength of 3.2 centimeters. The feed horn 31| is connected to a wave guide chamber 312 of rectangular cross-section of the order of onehalf inch by one inch, and having a convenient length, which is terminated in a crystal detector 315. The design of such crystal detectors is well known in the art and has been described by C'. F. Edwards in a paper Microwave Converters published in the November 1947 issue of the Proceedings of the Institute of Radio Engineers, pages 1181-1191.

I have discovered that distortion in the signal reflected back to the central station is largely reduced by locating a capacitative probe 313 in front of the crystal, and adjusting the longitudinal position and penetration of such probe in the guide 312 to obtain optimum reception and transmission.

The crystal detector 314, which may be biased one or more volts negatively by connection through the junction J and the choke coil 315 to the battery 315, is connected to a telephone speech and ringing circuit which includes the telephone transmitter 391, the telephone receiver 393, the bell 396 and its associated energizing circuit, the hook-switch S, and the relays Ri, R2, and R3.

The conventional relay R1 comprises the energizing coil 311 and the associated armature 313 which operates to close the normally open contact 31M, and the associated armature 315i which operates to close the normally open contact 31511.. The armature 318 is connected to the output of the detector 31d and the battery 315 through the junction J. The armature 319 is connected to the positive direct current source 391. The contact 3190i is connected to the telephone transmitter '5f-)i in series to ground with the primary coil of the iron-cored transformer 392, and the contact 378e is connected to the telephone receiver 383 which is in parallel with secondary coil of the transformer 392.

rlhe normally closed contact allah of the relay R1 is connected to the conventional filter 388 which is tuned to the frequency of the superaudible signal impressed on the carrier beam at the central station. The iilter 388 is connected through the conventional rectier Stil to the ringing coil 38) of the relay R2.

The relay R2, which may assume the form, for example, of the Weston sensitrol relay, model 755, type N, is preferably designed to have a sensitivity to power levels of 106 decibels below one watt. It comprises a ringing coil 333 and a reset coil 38|, which operates the armature 332 in different directions to make and break the contact 382er. The armature 332 remains actuated in one direction until attracted in the other direction. The contact 382m is connected to the positive direct current source 3537; and the armature 332 is connected to one terminal of the energizing coil of the relay R3.

The relay R3 comprises the energizing coil 383 connected in parallel to ground with the condenser 338, the associated armature 33t which operates to close the contact 38%, and the associated armature 385 which operates to make the contact 385er and break the contact 38519. lThe 'armature 38d is connected to ground, while he associated contact 3340, is connected to a parallel circuit one branch of which is connected to the energizing coil 377 of the relay R1 and the other branch of which is connected to the reset coil 38| of the relay Rz. r'lie armature 325 of the relay R3 is connected to a parallel circuit, one branch of which passes through the condenser 39:2 and the inductance B to the junction J.

The contacts 385e'. and 3&5?) are connected to the hook-switch S in a manner which will presently be described. The hook-switch S coinprises the mechanically coupled armatures S and 3&7, which are adapted, when the receiver is in place, to respectively close contacts 3Std and 387e, and when the receiver is released therefrom, to break the aforesaid contacts, and close contacts 38th and 387i. The make contact 38610 is connected to the coil 377 of the relay R1, the break contact 33M to the reset coil 38E, the make contact 337i) to the normally closed contact 385e of the relay R3, and the break contact 387er to the normally open contact 385@ of the relay R3. The armatures 336 and 387 are connected to the direct current battery 3&7.

In order to provide a better understanding of the operation of the circuits hereinbefore described, a complete call will now be traced.

Referring to Fig. 3, the subscriber at the calling station lifts the receiver 393 from the hookswitch S, which operation connects the battery 97 to the bell 3% through a circuit which includes the armature 337 and contact 387i) of the hook-switch S and the armature 385 and contact 385?) of the relay R3. The alternating voltage developed across the bell is applied to the crystal 374 through a circuit which includes the condenser 3M, the inductance 395, and the junction J. The aforesaid Variations in voltage which are impressed on the crystal detector 3M cause corresponding variations in the terminating impedance of the wave guide cavity 372. whereby microwave energy entering the wave 'i2 guide feed horn 37| from the antenna'reilector 370 is variously absorbed or reected back to the antenna 3W-37| for retransmission to the central station.

Thus, when the scanning carrier beam from the central office sweeps past the antenna l--lL a modulated echo is returned to the central office which indicates that a subscriber A wishes to make a call, and which automatically orients the scanning beam in As direction in the following manner. Referring to Fig. l, the renected carrier beam, modulated with the ringing signal is received by the central station antenna |||-|i2, passes through the connecting wave guide |53, the rotary joint |i7, and the wave guide arm i into the hybrid junction lfwi. At lthe junction Hifi, the received carrier wave is separated into two components, one of which is dissipated in the transmitting circuit and the other of which passes into the wave guide |42 where it is combined with aportion of transmit.- ter carrier and demoduiated by operation of the twin crystal detectors #it and M7 and the quadrature equalizer |53. vThe dernodulated ringing irrent from the-output of the quadrature equalizer circuit 253 passes through the low frequency amplifier ist, the transformer |53, filter |59V and rectiiier itil, to energize the relay YH6, which actuates its armatures H7, H8 and H9. The aforesaid operation breaks contact ll'la, thereby halting the motor lili, and closes contacts |I8a and liga, thereby applying the brake shoe |2602 to the brake drum m9 on the antenna shaft, and lighting the signal lamp |23, whereby the central station operator opens the switch |2| to disable the scanning operation for the period of the call, and closes the switch |22 to lock-operate the antenna brake.

The operator at the central office must now signal subscriber A that the central -oilice is ready to receive the call. In order to do this, the operator inserts the plug |35 in the jack |34 and presses the ringing key |32, whereby a superaudible ringing signal from the oscillator |3| passes through the transformer |33, the low frequency amplier |36, and the transformer |37 and is impressed on the carrier output in the microwave ampliiier-modulator lZ. The modulated carrier wave passes through the hybrid junction les and the connecting guides to the antenna iii-i l2, where it is transmitted in the form of a directed beam to the subscribing station A.

Referring again to Fig. 3, when the modulated ringing signal reaches'station A, it is received by the antenna EEES-371, from which it passes into the wave guide 372, where it is demodulated by the crystal detector 374. The demodulated signal passes through the junction J, the armature 378 and normally operated contact 37B?) of the relay R1 and'into the tuned lter 338. The output of the filter 32S passes lthrough the rectifier 389 and energizes the ringing coil 386 of the relay R2 to pull over the armature 382 and engage contact 33200, thereby applying the potential Ior the battery 97 to energize the coil 333 of the relay R3 and charge up the condenser 39." connected thereacross. This operation causes the armature 385 to break contact 3851i, thereby cutting off the circuit from the battery 397 to the bell 39E extending through the armature 387 and contact 3872) of the hook-switch S, thereby indicating to subscriber A that the central station is ready to receive a call.

Simultaneously, Athe subscriber As speech 13 transmitting and receiving circuit is connected as follows:

The relay R1 is energized by a circuit which is completed from the battery 391, through the armature and contact 386-3861) of the hookswitch S the energizing coil 311 and the armature and contact 33-35l1a to ground. The armatures 318 and 319 are thus actuated, breaking the contact 3182) and respectively connecting contacts 313:1. and 319m This operation connects the battery 391 to the circuit of the telephone transmitter 391, and the telephone receiver 393 to the junction J, whereby speech signals may either be impressed on the input of the detector 314 through a circuit which includes the transformer 392, or received from the output of the detector 314. In the former case, the unmodulated microwave carrier beam impressed on the antenna 31E-311 from the central station is modu lated with speech signals and reflected back to the central station in the manner described with reference to the ringing signals; while in the latter case, the speech modulations impressed -on the carrier beam at the central station are detected in the usual manner. The relays R1, R2 and R3 remain in operation during the conversation, inasmuch as the armature 382 remains engaged to contact 38205 as long as the hookswitch S is open, disengaging armature and contact 386 and 386m and thereby ldisconnecting the reset coil 381 from the battery 391.

Subscriber A now gives subscriber Bs number to the operator. Referring again to Fig. 1 of the drawings, the operator now orients a duplicate beam in the direction of subscriber B by f.)

operating a second set of central station apparatus as follows: By means of the handle 115a attached to the shaft 115 which operates the geardrive 1 IQ, the antenna 111-1 I2 is turned in the proper direction as indicated on the scale of the antenna direction indicator 163. By inserting the plug 135 in the jack 134 and depressing the key 132 in the second central station transmitter, the operator transmits a supersonic ringing signal to station B, in the manner described hereinbefore. This signal is intercepted by subscriber Bs antenna, and actuates the relays R1, R2 and Ra in station B, as described with reference to station A hereinbefore. Inasmuch as the hookswitch S is initially down, the bell 396 is connected to the battery 391 through a circuit which includes the armature and contact 385-38501 of the relay Rs, and the armature and contact 381- 381a of the hook-switch S. Simultaneously, the alternating voltage developed across the bell 396 is impressed on the crystal 314 through a circuit which includes the condenser 39d and the inductance 395, thereby providing a low frequency ringing signal which is transmitted back to the central station permitting the central station operator to hear the ringing of the subscribers bell.

Intermittent operation of the bell 396 is brought about in the following manner: Operation of the armature 384 to engage contact 38M. connects the circuit of the reset-coil 381 to ground, so that the armature 382 is attracted to disengage con-u tact 382a, causing the relay R3 to deenergize after a short interval required for the discharge of the condenser 399, and disengage contacts 381111 and 38511., thereby breaking the energizing circuit of the bell 396. This cycle is repeated as long as the central station operator continues to ring, or until subscriber B lifts the hook-switch S to an- Swer the call.

If the subscriber B lifts the hook-switch S while the bell 396 is still ringing, armature and contact 381-38101 are disengaged, thereby cutting off the battery 391, vwhich, operation causes the ringing to cease. If, on the other hand, the subscriber B lifts the hook-switch s during the interval when bell 366 is not ringing and relay 'R3 is open, the bell 396 is reconnected through a circuit which includes armature and contact 38E-385D of the relay R3 and armature and contact 381-3811) of the hook-switch S. The bell therefore continues to ring indicating to the operator that the central cnice must continue to transmit the ringing signal until armature 382 and contact 3820.

are engaged, thereby lock-operating relays R1..

R2, and R3 for the conversational period. When armature 382 and contact 3820!r are engaged, relay R3 is actuated connecting armature 384 to contact 381101 which causes relay R1 to opertae. 'Ihe contact 3851) is now disconnected and the bell stops ringing, indicating to the operator that the subscriber has answered.

At the central station, the operator disconnects the plug 156 from the jack 155, and the plug from the jack 134, thereby disconnecting the telephone transmitters and receivers in each of the respective centralstation apparatus, and in their steads inserts jumpers which connect the receiving pack |55 of one of said apparatus to the transmitting jack |311 of the other apparatus, and vice versa. This conditions the system for speech transmission and reception between subscribing stations A and B.

At station A, speech signals impressed on the transmitter 39| induce currents in the secondary coil of transformer 392 which pass through the armature and contact 318-318a and the junction J and are impressed on the crystal 31d, changing its impedance, and thereby modulating the carrier beam which is received from the central station and reflected back thereto from the wave guide 312 and the antenna 319-311.

The modulated reflected carrier beam is intercepted by the antenna 111-112 in the rst central station apparatus, and passed therefrom into the receiver in the same manner as the ringing signals, where it is homodyned with a component of unmodulated carrier wave. The speech output is conducted from the output of the audioamplifier 154 in the first central station apparatus through the jack |55 and connecting jumper to the speech input jack 134 of the second central station apparatus. The speech signals thus impressed on the second apparatus pass through the audio amplifier 136 and into the microwave amplier-modulator 102, where they serve to modulate the microwave carrier beam which is radiated from the second antenna 1 1 1-1 12 in the direction of station B.

When the speech modulated carrier beam from the central station is intercepted by the antenna 3111-311 at station B, the modulations thereof are detected by the crystal 314 in the wave guide 312, and impressed on the telephone receiver 393 through a circuit which includes the junction J and the armature and contact 318-318-a of the relay R1. In a similar manner, subscriber` Bs speech signals are transmitted to subscriber A.

At the termination of the call, the receiver is replaced and hook-switch S depressed by one or both parties, thus causing one or more ringing signals to be transmitted to the central station, during the period of discharge of the condenser 398, for Ywhich interval the relay R3 remains operated and connects the battery 391 to the bell est; whereey'aiewfrequecysigirenstrapsmtted asf flese'rilmd-l her'ein'before'.-v I-l1ese-sigii`alsy are reni"l ceivedin' the manner" hereinbefore described by one ori' both set'sf offcentrallstation apparlatu's,'rr Wherein'the respective" relaysf |H'areactiia'rted);- 5" lig-l'ltingv the signal "lahip's |23 af1`d."noty-ing3 theI central station operator of the termination of` the call.` The operato'rremovesfth'econnecting? jumpers from the' speech circuitsi'a'n'din' each" oflthe' sets ofi-apparatus, opensfthe switchlf'to" release the-brake-shoefl 26afro1`nthe brake-drum le'f eri ithe'fentenne shaft; and eio'sesf'thef Switch l |2'l to reestablish the scanning operations oftheantennas.y

For convenience of' il'll'istration;A i the system v-of the presentinVention-has been' described structur'ailyY an'din' operationwith referenceto par-A' tiul'arl sets' of' apparatus-iin* the' centrali-stationiandthe suh'scribers 'staft'ons,4 such* asfshwinv` in Figs. 1; 2, and 3 ef'the-'drewm'gse It Winv be v apparent to tlrc'ase skilled inf-the artf tlatthe ysys-' tem of the present invention'isflnt-coniined'tor any vof fthe particular circuit f arrangements shown or'an'y'of thespeciic elements thereon-andmayA take numerous'alternativeforms-such, fo'r'ex- 25` ample, as the central station 'and lsu'bl'scrib'ers cir-` cuit-shown in Fig. 4; which comprises a system in accordance Withthe present invention in 'which the 'transmitted and" received 'carrier"beejrns` are* polarized in Ydifferent planes.

Inaccordanceiwith 'the arrangement shown in Fig, v 4;' thecen'tral ofl'ce'transmits' afvertically polarized wave which iseither mod'lated'with speech or'ringing signals, at`the central station 0r"ur'imodulated. If 'the Wave arrives at'thesub- 30 seribersV stationn modulated', it is'th'er'e dernoduij lated 'by'mean's offa bridge-combination vof four crystals connected in series-parallel. If the Wave arrives unmo'dlated and if th`e"subscrib`erirr` presses a speech signal across the vtivo horizontal terminals ofthe bridge, echo side-bands varegenerated With a horizontal'polarizatiomvand'reilected backl to the central station. Such a systeur-in which communication in opposite direcf ticn's is carried on by means ofperpendicularly polarized waves, reducesthe amount of `side-tonepresent at either station.

Referringto Figi 4, the -microwavegenerator unit lill whichmayrassumeany'4 of the forms well known inV the art has a'sel'f--contained rectier-and hum l-terso'that it`is va-hle to operatedirectly vfrom4` a kconventional source of -Jalternating current power/i 3 -connectedj thereto/ through the slip ringcorrtactsMT-Ma and 428-428af The generator il I; oscillates with 'affwavelengthu of; for example, I1Scentimeters-W-ith-a pow-croutput ofthe order of 50-Watts-- The vertically pollarized output of the microwave'generator] isconducted Y through 'a connectingA wave'gu-ii and through ther grating i402 comprising I aserie's" ofcopper' Wires; such' a's described. withreferl eneeito Fig.y iofny application "S1 N; 540,533; filed 'June 16,R 1944,l issued?asUliid'StafesfPaiF ent 2,441,598,1Vlay'18, 1948, Which is 'sulostait`ally' transparent to the ve'rti'all'y"v polarized" L incident guided :wai/es butV4 which 'constitutes' :a substan'l* tially 'perfect reflector or"t guided Way'es'haVing horizontal polarization.' After'passing through the -grating @Si the'v Wave'ietr's' the "Wave" guideV 70A chamber 403a-which isfa' continuation o fth'e' wave Aguide 'ac3 "and whieh'my;'fefexamlejhave aY eresseseeuen of sibyatee'nti'ieters'and' a length' ofVA the; order of 20" centimeters The' 'output `of chamber fiw'iseenne'eted"wreint'a dirc'' 0 low truncated pyramid flaring from aniTA in'nef nite-c1 applicati-n serial secondary' ofj transformer'- MD, I'a j jackand plug connection 43Q-2-435;` andthe slip ring contact -flt'bf arer also connected -to receive'r a superaudible ringingcurrent from' the-oscillatoif'fii.V T herconnectinggcircuit between the ringng oscillator 43|- and the terminals 404m,-

40|b` the normally openv ringing L push buttonl'f, the transformer 4313;, the jack and Aerese the ether-pairhoffterminms 4040.404@- ofgtljleaforesaid bridge'll whichare substan-` tiallyalignedlrorizontallyfin'the wave guide-4ll3a, are` cmqmect -e d Y the Aslip-ring' conta-cts '40k-4080.

A amplierf- 454 f areassumed-*to besubstantiallysimilar Y to the circuitsm connected to the" output of amplifier |54 in Fig. l similar circuit elements the gear drive M0 'adapted to produce a scanning vertically polarized carrier beam in the manner of the hereinbefore described central station equipment.

At the subscribers station the directional horn antenna 4H, which is of a type similar to the transmitting antenna horn 405, is connected to a wave guide 412, in which is also disposed a series-parallel bridge circuit oi. crystal rectifiers 414. To the right of bridge 414 the end of the wave guide 412 is closed by a conducting metal plate 412m disposed substantially a quarter of a guide wavelength from the bridge circuit 414.

The lower of the two substantially vertical terminals 414s of bridge 414 is connected to ground. The upper vertical terminal 4142 is connected to armature 478 of relay R1 Which is identical to and serves the same function as relay Ri oi Eig. 3. Armature 418 makes contact to contact 418i) which is connected to high-pass filter 488 which is substantially identical to and serves the same purpose as filter 388 of Fig. 3. The circuit to the right of the section line A-A is identical to the circuit to the right of section line AMA of Fig. 3 and has been omitted for simplicity in description.

When relay Ri is closed the vertical terminal of the bridge 414 is connected to the telephone receiver 493 through armature 418 and contact 4180. oi relay R1.

The two horizontal terminals 414e, 414d of bridge 414 are connected to the secondary winding of transformer 41E. The primary of transformer 416 is connected to a circuit which includes the telephone transmitter 491 and relay contact 419s. Relay armature 419 is connected to the circuit to the right of section line A-A in the manner described in connection with Fig. 3. When armature 419 engages contact 419e, speech signals impressed upon telephone transmittel @Si are applied to the horizontal terminais of the bridge 414, which signals intermodulate with the carrier wave already impressed upon the bridge and generate horizontally polarized speech side-bands in the wave guide chamber 412. The horizontally polarized speech sidebands are transmitted in the direction of the central office through the antenna horn 41|. Ringing signals originating at the subscriber station in the circuit to the right of section line A-A are impressed upon the horizontal terminals of the bridge through a circuit Which includes the inductance 493, the capacitance 494 and the transformer 416.

It will be apparent to those skilled in the art that the systems described in the foregoing pages may take numerous other forms not shown herein which are within the scope of the present invention and that embodiments of the present invention are accordingly not limited to the use of any particular element or combination of elements shown by Way of illustration herein.

What is claimed is:

l. A radiant energy system comprising in cornbination a main station, a plurality of satellite stations, means at said main station for generating a frequency modulated carrier wave, means in energy transfer relation with said generating means for radiating said carrier Wave in a directional beam, driving means coupled to said radiating means for causing said beam to repeatedly scan an area which includes said satellite stations, means connected to said driving means for controlling said driving means to fix the orientation of said beam in the direction of a selected satellite station, and means at said selected sat- Lif ellite station and at said central station for utilizing lsaid frequency modul-ated carrier beam for the two-way transmission of intelligence, said means including an energy reflecting and absorb.- ing circuit tuned to frequencies within the frequency band of said carrier wave and responsive to impressed modulating signals to vary the reflection of carrier wave energy therefrom.

2. A radiant energy system comprising in combination a main station, a plurality of secondary stations, means at said main station for generating and radiating a directed beam of frequency modulated carrier waves, driving means coupled to said generating and radiating means for causing said beam to repeatedly scan an area which includes said secondary stations, means at said secondary stations for reflecting and absorbing components of said beam, modulating means at said secondary stations connected to said refleeting and absorbing means for controlling the reflection of beam energy by said last-named means, calling means at said secondary stations in energy transfer relation with said modulating means for impressing a calling signal on the reflected. component of said beam, detecting means at said main station responsive to the reilected component of said beam, means connested to said detecting means and responsive to said calling signal for controlling said driving means to fix the orientation of said frequencymodulated beam in the direction of said calling secondary station, means connected to said generating and radiating means at said main station for modulating said frequency modulated radiated beam in accordance with speech signals, speech receiving means connected to said reflecting and absorbing means at said secondary stations, speech transmitting means connected to the modulated means at said secondary stations, speech receiving means connected to the detecting means at said main station, and calling means in energy transfer relation with the modulating means at said main station for impressing a calling signal on said frequency modulated radiated beam.

3. A system in accordan-ce with claim 2 in which said transmitted carrier beam is frequency modulated in accordance with a repetitive sawtooth cycle, and wherein said detecting means is connected to said generating means to receive a component of transmitted energy, and wherein said detecting means comprises means for detecting the intermediate frequency wave derived from beating the reected component of said beam with a component of transmitted carrier energy.

4. A system in accordance with claim 3 in which said speech receiving means at said central station comprise a plurality of units correspending to satellite stations at different distances, and each tuned to a different band of intermedate frequencies.`

5. A radiant energy system comprising in combination a main station, a plurality of satellite stations, means at said main station for gen'- erating and radiating a directed beam of carrier Waves, driving means coupled to said generating and radiating means for causing said beam to repeatedly scan an area which includes said satellite stations, means connected to said driving means for controlling said driving means to fix the orientation of said beam in the direction of .a selected satellite station, and means at said selected satellite station for utilizing said carriet beam for the two-way transmission of inteiligence, said means including an energy reflecting and absorbing circuit responsive to impressed modulating signals to vary the reflection of carrier wave energy therefrom.

6. A radiant energy system comprising in combination a main station, a plurality of secondary stations, means at said main station for generating a carrier Wave, means in energy transfer relation With said generating means for radiating said carrier Wave in a directional beam, driving means coupled to said radiating means for causing said beam to repeatedly scan an area which includes said secondary stations, means at said secondary stations for reflecting and absorbing a component of said beam, modulating means at said secondary stations connected to said reflecting and absorbing means for controlling the components of the energy of said beam which are respectively reected and absorbed, calling means at said secondary stations in energy transfer relation with said modulating means for impressing a calling signal on the reiiected component of said beam, detecting means at said main station responsive to the reected component of said beam, means connected to said detecting means and responsive to said calling signal for controlling said driving means to fix the orientation of said beam in the direction of said calling secondary station, means connected to said generating and radiating means at said main station for modulating said radiated beam in accordance with speech signals, speech receiving means connected to said reiiecting and absorbing means at said secondary stations, speech transmitting means connected to the modulating means at said secondary stations, speech receiving means connected to the detecting means at said main station, and calling means in energy transfer relation With the modulating means at said main station for impressing a calling signal on said radiated beam.

7. A system in accordance with claim 6 in which the means at the secondary station for reiiecting and absorbing components of said carrier beam comprise a directional antenna, a tuned circuit connected to said antenna, and means comprising a varistor coupled to said tuned circuit to present a varied impedance to the energy in said tuned circuit.

8. A system in accordance with claim 'i in which the means at the secondary stations for reiiecting and absorbing a component of said carrier beam comprises a directional antenna having a Wave guide feed, and a crystal rectiiier disposed in said Wave guide and responsive to signal currents to present a varied impedance to the energy in said Wave guide.

9. A radiant energy system comprising in combination a central station which includes a source of carrier Waves, a receiver responsive to receive said carrier waves, an antenna constructed to radiate said carrier Waves in a directional beam, means for connecting said source and said receiver to said antenna, driving means mechanically coupled to said antenna to cause said beam to repeatedly scan a predetermined area, a modulating circuit connected to said carrier wave source, said modulating circuit including speech transmitting means and a source of calling signal current, a detecting circuit connected to said receiver, said detecting circuit including speech receiving means and calling signal responsive means, said -calling signal responsive means connected to means to control the operation of said antenna driving means, a plurality of secondary stations'positioned in the scanning path of the beam radiated'by said central station, each of said secondary stations including a directional antenna responsive to said beam, energy reecting and absorbing means connected to said antenna, controlling means connected to said reflecting and absorbing means to control the reection and absorption of energy therefrom, speech transmitting and receiving means, a source of calling signal current, a calling signal annunciator, means connected to said controlling means and responsive to a calling signal from said central station for actuating said signal annunciator and for connecting said calling signal source to said controlling means, and switching means for disconnecting said calling signal source from said controlling means and for connecting said speech transmitting and receiving means in its stead.

l0. In a radiant energy system, a satellite station comprising in combination a directional antenna, a tuned circuit responsive to reflect and absorb carrier Wave energy received by said antenna, modulating means coupled to said tuned circuit for varying the amounts of carrier Wave energy reflected and absorbed thereby in accordance With impressed signal currents, a source of calling signal current, means responsive to a calling signal from a remote point to connect said calling signal source to said modulating means, intelligence transmitting and receiving means, and switching means for .disconnecting said calling signal source from said modulating means and for connecting said intelligence transmitting and receiving means in its stead.

ll. In a radiant energy telephone system, a satellite station, comprising in combination a directional antenna, a Wave guide responsive to reiiect and absorb carrier Wave energy received by said antenna, modulating and detecting means disposed in said wave guide for varying the amounts of carrier wave energy reiiected and absorbed by said wave guide in accordance with impressed signals, a source of calling signal current, speech transmitting and receiving means, and means for connecting said source and said transmitting and receiving means to said modulating and detecting means.

12. In a radiant energy telephone system comprising in combination a main station and a plurality of secondary stations, said main station including means for generating and radiating a beam of carrier Waves, and said secondary stations including means for modulating and reflecting the carrier wave beam from said main station, the improvement comprising in combination driving means connected to said generating and radiating means at said. main station for causing said beam to repeatedly scan an area including said secondary stations, means connected to said modulating means at said secondary stations for impressing a calling signal on said reflected beam, calling signal receiving means at said central station, and means operative on said driving means at said main station for fixing said beam in the direction of a calling secondary station.

13. A system in accordance with claim 12 in which said means operative on said driving means is connected to said calling signal receiving means and operates in response to said calling signal.

14. A system in accordance with claim 12which includes a calling annunciator, said annunciator connected to said calling signal receiving means and operative in response to said calling signal 21 to notify the central station operator of an incoming call.

15. A telephone system comprising in combination, a central station and a plurality of satellite stations, radio transmitting and receiving means at said central station, means connected to said transmitting means to propagate a directional beam of radio frequency carrier waves, means to cause said beam to repeatedly scan through a predetermined arc, radio reflecting means at each of said satellite stations positioned to lie in the path of said scanning beam, means responsive to a signal from a particular one of said satellite stations for halting said scanning motion and directing said beam toward said particular satellite station, and means connected to said radio reecting means for modulating said reected carrier waves in accordance with desired signal currents whereby said signal modulated carrier waves are reflected back to said central station and received by said radio receiving means.

16. A radiant energy system for the transmission of intelligence comprising in combination a central station and a plurality of satellite stations, radio transmitting and receiving means at said central station, means connected to said transmitting means to propagate a directional beam of radio frequency carrier waves, driving means to cause said beam to repeatedly scan through a predetermined area which includes said satellite stations, means at each of said satellite stations for reflecting a portion of the energy of said beam back to said central station, means connected to said reflecting means for changing the polarization of the reflected beam with respect to the transmitted beam, means responsive to a reflected signal from a calling satellite station for halting said scanning motion and directing said beam toward said calling station, and means connected to said reecting means and said polarization changing means for modulating said reflected carrier waves in accordance with desired signals whereby said signal modulated carrier waves are reected back to said central station and received by said radio receiving means.

17. A communication system comprising in combination a central station and a plurality of satellite stations, a generator of carrier wave oscillations at said central station, means comi prising a circuit connected to said generator for varying the frequency of said oscillations through a substantially repetitive saw-tooth cycle, means at said central station responsive to said frequency varied oscillations to radiate a beam of energy in the direction of a selected one of said satellite stations, means at said selected satellite station for modulating the energy of said beam in accordance with impressed signals and redirecting said modulated beam back to said central station, receiving means at said central station disposed to receive the signal modulated beam from said selected satellite station, means at said central station comprising a mixing circuit in energy transfer relation with said generator and said receiving means for combining the oscillations of said received beam with a component of oscillations from said generator, a plurality of beat detecting means selectively responsive to the different beat frequency output currents from said mixing circuit, a plurality of indicating means respectively connected to said diierent beat-frequency detecting means for indicating satellite stations at respectively different distances, and a plurality of speech receiving means respectively connected to said different beat detecting means.

SLOAN D. ROBERTSON.

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